For manufacturing semiconductor devices, it is necessary to conduct the thinning process for reducing the thickness to 100 micrometers or less and/or stress relief and also to form a gettering layer. Here, the stress relief means removal of damage on a ground surface of a wafer caused by grinding, and the gettering means trapping of contaminant metal ions in the environment.
Conventionally, the gettering layer has been formed by dry polishing using a polishing pad of a fixed-abrasive grain type or a grinding wheel. For example, Patent Document 1 discloses that a strained layer which provides sites for the gettering of metal impurities is formed by polishing a back surface of a semiconductor wafer by using fixed abrasive grains. Meanwhile, Patent Document 2 discloses a method for carrying out a gettering process by grinding a back surface of a semiconductor device with a grinding wheel obtained by bonding diamond abrasive grains having a particle diameter of 4 μm or less with a bonding material. Patent Document 3 discloses a processing method preferable for forming a gettering layer by polishing a back surface of a wafer with a polishing member in which abrasive grains having an average particle diameter of 5 μm or less are dispersed.
However, all of the grinding wheel and the like used in Patent Documents 1 to 3 are designed for polishing (dry-polishing) a semiconductor wafer without using a slurry. Hence, the formation of a gettering layer by the dry polishing of each of Patent Documents 1 to 3 is disadvantageous in that dust is formed during the polishing. In addition, when the polishing member of Patent Document 3 is used, it is necessary to conduct the thinning process and stress relief by conducting polishing or etching in steps before the formation of the gettering, which causes problems in terms of operation efficiency and costs.
Meanwhile, regarding a polishing pad used not for a dry polishing method but for a wet polishing method, for example, Patent Document 4 discloses a polishing pad in which abrasive grains and alkali particles are included in a foamed polyurethane. With the polishing pad, a gettering layer is formed on a back surface of a wafer by using pure water as a polishing liquid.
However, the polishing pad described in Patent Document 4 is made of foamed polyurethane, and hence has closed cells. Thus, the polishing pad tends to have a problem of clogging by polishing dust or the like. In addition, the polishing pad described in Patent Document 4 is disadvantageous in that the resin used for the foamed polyurethane has a high hardness and thus a thinned wafer tends to be cracked.